Which of the following vertebrates has the most simple structured heart

Analyze the question: The question asks which vertebrate group has the simplest heart structure. This is about the number of chambers in the heart of each vertebrate class.

Explain the concept: Vertebrate hearts become more complex as we move from aquatic to land animals:

| Vertebrate | Heart chambers | |---|---| | Fish | 2 chambers (1 atrium + 1 ventricle) | | Amphibian | 3 chambers (2 atria + 1 ventricle) | | Reptile | 3 chambers (2 atria + 1 partially divided ventricle; crocodiles have 4) | | Mammal (and birds) | 4 chambers (2 atria + 2 ventricles) |

Step-by-step solution:

1. Fish have a two-chambered heart: one atrium and one ventricle.
2. Blood flows in a single circulation: heart → gills → body → heart. It passes through the heart only once per complete circuit.
3. Amphibians and reptiles have three chambers, with a double circulation (though mixing of blood occurs).
4. Mammals have four chambers, with complete separation of oxygenated and deoxygenated blood.
5. Fewer chambers means a simpler structure, so the fish has the simplest heart.

Answer: B. Fish

Exam tip: Remember the sequence 2-3-3-4: Fish = 2, Amphibians = 3, Reptiles = 3 (almost 4), Mammals = 4. Think "Few Are Really Many" to recall the order from simplest to most complex. Whenever JAMB asks for the "simplest" or "most complex" heart, count the chambers.